1. Field of the Invention
Embodiments of the present invention relate generally to a semiconductor memory device. More particularly, embodiments of the invention relate to a flash memory device and an associated cache read operation.
A claim of priority is made to Korean Patent Application No. 2005-27659, filed on Apr. 1, 2005, the disclosure of which is hereby incorporated by reference in its entirety.
2. Description of Related Art
Semiconductor memory devices can be broadly classified into two categories: volatile memory devices and non-volatile memory devices. Volatile memory devices include, for example, random access memory (RAM). A RAM typically stores information by either setting a logic state of a flip-flop, as in a static random access memory (SRAM) or by charging a capacitor as in a dynamic random access memory (DRAM). Volatile memory devices can only store data as long as they receive power. They lose stored data when their power supplies are interrupted.
In contrast, non-volatile memory devices such as masked read only memory (MROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM) and electrically erasable programmable read only memory (EEPROM) continue to store data even when their power supplies are interrupted. Non-volatile memory devices may store data permanently, or they may be reprogrammable, depending upon the fabrication technology used to create the devices. For example, EEPROMs, which are programmed electrically, can be repeatedly reprogrammed. However, PROMs, which are programmed by blowing fuses, can only be programmed once.
Because non-volatile memory devices retain stored data even when power is cut off, they are commonly used to store microcode in a wide variety of applications in the computer, avionics, telecommunications, and consumer electronics industries. Some systems also use a combination of a volatile memory and a non-volatile memory to provide non-volatile storage capability in high performance applications. For example, a non-volatile SRAM (nvRAM) loads data from a non-volatile memory to a SRAM to run a program. When the program runs, the data may be updated in the SRAM. Then, to preserve the data when power is disconnected, the data is stored back in the SRAM. In addition, dozens of special memory architectures have evolved which contains some additional logic circuitry to optimize their performance for specific tasks.
Unfortunately, a user cannot freely program MROMs, PROMs, and EPROMs. For instance, erasing an EPROM requires applying an ultraviolet (UV) light to the memory's surface for an extended period of time. In contrast, EEPROMs can be electrically erased and programmed in an efficient manner. As a result, EEPROMs are commonly used to supplement conventional memory systems or to store data that requires non-volatile storage, but undergoes periodic updates.
One of the most popular forms of EEPROM currently is flash memory. Flash memories have a higher integration density than most other types of EEPROM and as a result, they are often used to provide mass storage. Flash memory is typically divided into two categories depending on the type of logic gate used in each memory cell. These categories are NAND flash memories, and NOR flash memories. NOR flash memories generally provide faster read times than NAND flash memories, but they are more expensive, they are slower to write and erase, they can withstand less programming, and they take up more space. As a result, NOR flash memories are generally used to store program code, while NAND flash memories are used to provide mass data storage.
Where a memory system includes both NAND flash memory and NOR flash memory, control circuitry for the memory system becomes complex. One way to address this complexity is through an integrated memory architecture. In an integrated memory architecture, program code that is ordinarily stored in a NOR flash memory is stored in a OneNAND™ flash memory together with data previously stored in a NAND memory. An exemplary OneNAND™ flash memory is disclosed in the data book of Samsung Electronic Co., Ltd., published in September, 2003, which is hereby incorporated by reference. In the integrated memory architecture, data is transferred between the OneNAND™ flash memory and a host system.